Cmut assembly with acoustic window

ABSTRACT

In some examples, a capacitive micromachined ultrasonic transducer (CMUT) apparatus includes one or more CMUTs formed on a CMUT substrate to have an operational direction facing away from the CMUT substrate. As one example, the one or more CMUTs may include a plurality of CMUT cells arranged in groups to form CMUT elements, and a plurality of the CMUT elements may be configured as an array on the CMUT substrate. An acoustic window is disposed over the one or more CMUTs and may contact an external medium. For instance, the acoustic window may be positioned to pass acoustic energy to or from the one or more CMUTs in the operational direction. A coupling medium may be disposed between the CMUTs and the acoustic window to couple acoustic energy between the one or more CMUTs and the acoustic window.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 13/467,638, filed May 9, 2012, which isincorporated herein by reference.

TECHNICAL FIELD

Some implementations herein relate to acoustic transducers andtransducer arrays, such as capacitive micromachined ultrasonictransducers (CMUTs) and CMUT arrays that may be employed with anacoustic window.

BACKGROUND

Electrostatic actuators and ultrasonic transducers may be used forvarious applications in a variety of media including liquids, solids,and gas. For instance, ultrasonic transducers are commonly used inmedical imaging, such as for diagnostics and therapy. Other uses mayinclude biochemical imaging, non-destructive evaluation of materials,sonar, communications, proximity sensing, gas flow measurements, in-situprocess monitoring, acoustic microscopy, underwater sensing and imaging,and numerous other practical applications.

A typical CMUT may include at least two electrodes with a transducingspace (e.g., a separation gap) between the two electrodes that allowsone of the electrodes to be physically displaced toward and away fromthe other electrode during operation. On the other hand, a typicalpiezoelectric transducer, such as those using lead zirconate titanate(PZT) may include a ceramic disc of piezoelectric material that developsa voltage across two of its faces when compressed (such as for sensorapplications), or that physically changes shape when an externalelectric field is applied (such as for actuator applications includingultrasonic applications).

PZT transducers may sometimes be used with an acoustic lens that isplaced on the front surface of the PZT transducer to shape the acousticbeam produced by the PZT transducer. A commonly used acoustic lensmaterial for a PZT-based ultrasonic transducer in medical imaging is RTVsilicone rubber (RTV). However, the acoustic loss in RTV may besignificant, especially when the PZT transducer is operated at arelatively high frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is set forth with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical items or features.

FIG. 1A illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window according to some implementations.

FIG. 1B illustrates a cross-sectional view of an example CMUT apparatusincluding a CMUT array and an acoustic window according to someimplementations.

FIG. 2 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window and a CMUT (or CMUT array) having afocusing capability according to some implementations.

FIG. 3 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window and a sealed coupling medium accordingto some implementations.

FIG. 4 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window and a sealed coupling medium accordingto some implementations.

FIG. 5 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a planar shape and a uniformthickness profile according to some implementations.

FIG. 6 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a convex shape and a uniformthickness profile according to some implementations.

FIG. 7 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a concave shape and a uniformthickness profile according to some implementations.

FIG. 8 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a tilted planar configurationand a uniform thickness profile according to some implementations.

FIG. 9 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having multiple tilted facets and auniform thickness profile according to some implementations.

FIG. 10 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having gradually varied patterns orundulations and a uniform thickness profile according to someimplementations.

FIG. 11 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a planar shape with amultiple-step non-uniform thickness profile according to someimplementations.

FIG. 12 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a planar shape with amultiple-step non-uniform thickness profile on multiple sides accordingto some implementations.

FIG. 13 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window having a planar shape with a graduallyvaried non-uniform thickness profile according to some implementations.

FIG. 14 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window that forms a concave lens with a flatsurface adjacent to the coupling medium according to someimplementations.

FIG. 15 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window forming a concave lens with a curvedsurface adjacent to the coupling medium according to someimplementations.

FIG. 16 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window forming a convex lens with a flatsurface adjacent to the coupling medium according to someimplementations.

FIG. 17 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window forming a convex lens with a curvedsurface adjacent to the coupling medium according to someimplementations.

FIG. 18 illustrates a cross-sectional view of an example of a CMUTapparatus with an acoustic window made of multiple layers of materialsaccording to some implementations.

FIGS. 19A-19C illustrate an example of a CMUT apparatus with an acousticwindow having multiple layers of materials according to someimplementations.

FIGS. 20A-20C illustrate an example of a CMUT apparatus with an acousticwindow having multiple layers of materials according to someimplementations.

FIG. 21 illustrates a perspective view of an example implementation of a1D CMUT array with an acoustic window according to some implementations.

FIG. 22 illustrates a cross-sectional view of an example implementationof a CMUT array with an acoustic window according to someimplementations.

FIG. 23 illustrates an example of an acoustic window and a supportstructure according to some implementations.

FIG. 24 illustrates an example of an acoustic window and a supportstructure according to some implementations.

FIG. 25 illustrates an example of an acoustic window and a supportstructure according to some implementations.

FIG. 26 illustrates an example of an acoustic window and a supportstructure according to some implementations.

FIG. 27 is a flow diagram illustrating an example process forconfiguring and using a CMUT with an acoustic window according to someimplementations.

DETAILED DESCRIPTION

This disclosure includes techniques and arrangements for a CMUTapparatus that may include an acoustic window. In some implementations,the CMUT (or the CMUT transducer elements of a CMUT array) may produce afocused acoustic output similar to what might be achieved through use ofan acoustic lens. For example, through micromachining techniques, one ormore CMUTs or one or more CMUTs as elements in an array can beconstructed to have a desired shape to enable focusing of an acousticbeam transmitted by the CMUT(s). For instance, the CMUT in someimplementations may be curved or otherwise shaped to focus emittedacoustic energy on a focal location and/or to achieve a desired acousticbeam profile. Thus, in some implementations, an acoustic lens may not beused or needed for focusing the acoustic beam. Accordingly, in theexamples in which an acoustic lens is not used to focus the acousticenergy, the acoustic velocity or acoustic losses in the protectionstructure, such as those that may occur with RTV, are no longer aprimary concern when designing a protective exterior contact surface orwhen selecting a material to use as the exterior contact material.Consequently, some implementations herein include a CMUT apparatushaving an acoustic window that does not incur a substantial acousticloss, unlike the lens materials commonly employed with PZT-basedtransducers.

In some examples, the acoustic window may have a substantially uniformthickness profile, i.e., a uniform thickness in cross section. The shapeof the acoustic window with the uniform thickness profile can be flat,convex, concave, and so forth. The acoustic window can also be designedto have periodic or non-periodic shapes or patterns, while still havinga uniform thickness. For example, the acoustic window may include atleast one structural feature to reduce or minimize acoustic reflection.Additionally, in other implementations, the acoustic window may have anon-uniform thickness profile. For instance, the thickness profile caninclude multiple-step patterns or continually varied patterns. Thepattern of the non-uniform thickness profile can be periodic ornon-periodic, and the patterns may be designed or selected to minimizeacoustic reflection. Further, the shape of the acoustic window with thenon-uniform thickness profile (or pattern) can be flat (planar), convex,concave, or the like.

In some examples, the thickness profile of the acoustic window may beshaped as a convex lens or a concave lens so that the acoustic windowmay focus an acoustic output from the CMUT(s) or focus an acoustic inputtoward the CMUT(s). Further, periodic or non-periodic uneven surfacepatterns may be formed on the surface of the acoustic window to providevarious effects on the acoustic energy, such as reducing reflectiontoward the CMUT(s). In some implementations, the acoustic window alonemay provide a focusing functionality, while in other implementations,the coupling medium alone may provide a focusing functionality. In stillother implementations, both the acoustic window and the coupling mediummay together provide a focusing functionality, such as in the form of acompound acoustic lens. Additionally, in some examples, the acousticwindow may have multiple layers, one or more coatings, and/or or one ormore transition regions. The multiple layers, coatings, or transitionregions may have a thickness and/or impedance arranged to minimizeacoustic reflection.

In one implementation, the CMUT array may include a common electrode fora plurality of CMUTs in the CMUT array. The common electrode may beclose to or adjacent to a coupling medium and a protection layer may beformed on the CMUTs in the CMUT array. In some examples, the CMUTs maybe coated with the protection layer so that one or more electrodes ofthe CMUTs do not directly contact the coupling medium, such as in thecase in which the coupling medium is a conductive or semiconductivematerial. Thus, in some cases, the protection layer may insulate one ormore CMUT electrodes from the coupling medium.

As mentioned above, a CMUT differs structurally from a PZT transducer,employs different construction materials, and may provide differentacoustic properties. A basic structure of a PZT transducer may be asandwich-type structure including a matching layer, a PZT layer, and abacking layer. On the other hand, in some examples, a CMUT may beessentially constructed as a parallel capacitor with a movableelectrode. Further, because the CMUTs herein typically have a much loweracoustic impedance than a PZT transducer, the CMUTs herein may notutilize or require a matching layer. The CMUTs herein may be fabricatedon a CMUT substrate, such as silicon or other suitable substrate, andmay be made flexible enough to form a curved shape to provide a focusingcapability. Accordingly, the CMUT apparatuses herein may include CMUTsand CMUT arrays that are arranged to focus on an exterior focallocation.

The CMUT apparatuses herein may be employed with any type of CMUT orarray of CMUTs. For instance, there are many different possible CMUTdesigns (e.g., CMUT with flexible membrane, CMUT with embedded spring orsurface plate, etc.), and accordingly, implementations herein are notlimited to the particular CMUT structures shown and described. In thedisclosure, a basic structure of a CMUT is used to illustrate thedisclosed apparatus structures, arrangements and techniques, but thesame apparatus structures, arrangements and techniques may be used inconjunction with any type of CMUT or CMUT array, including, but notlimited to, flexible membrane CMUTs and embedded spring CMUTs (ESCMUTs),etc.

In some examples herein, a CMUT may include a first electrode and asecond electrode separated from each other by a transducing space (or anelectrode gap) so that a capacitance may exist between the electrodes. Aspring member (e.g., a resilient flexible membrane or a spring layer)may support one of the electrodes to enable the two electrodes to movetoward or away from each other. For instance, in a flexible membraneCMUT, the spring member may be a flexible membrane directly supportingone of the electrodes. Alternatively, in an ESCMUT, the spring membermay be a spring layer supporting an electrode on a plate, which issuspended from the spring layer by spring-plate connectors. In addition,each CMUT may be made up of one or more CMUT cells. In some examples,multiple CMUT cells may make up a single CMUT and the cells may be ofthe same design as one another, or in other examples, the design of thecells may be different from one another.

In general, one CMUT or multiple CMUTs can be used to perform afunction. In the case that multiple CMUTs are used, the multiple CMUTsmay be arranged to form a CMUT array and each CMUT of the multiple CMUTsmay be an element in the CMUT array. Thus, a CMUT array (e.g., a 1D,1.5D, 1.75D, 2D array, etc.) can be formed by arranging multiple CMUTsin a pattern, such as in a line, in a grid, in cross pattern, and soforth Implementations herein are not limited to any particular patternor arrangement of CMUTs in a CMUT array. In some examples, each CMUT inthe CMUT array may be a fully functional CMUT transducer that may beindividually and independently addressable, i.e., an identifiableelectrical output may be received therefrom or an identifiableelectrical input may be provided thereto. Alternatively, in someexamples, two or more CMUTs of multiple CMUTs included in a CMUT arraymay be commonly addressed CMUTs.

A CMUT or CMUT array may be packaged on a packaging substrate formechanical support and/or to enable electrical connections. In somecases, the packaging substrate also may be used as an acoustic backinglayer or may serve one or more other functions. The portion of thepackaging substrate contacting the CMUT may be made of the material withhigh acoustic loss and the acoustic impedance close to that of the CMUTsubstrate, e.g. silicon or glass. In some examples, the packagingsubstrate may include some metal pads/traces (wires) for electricalconnections, or may include a printed circuit board (PCB—e.g., flexiblePCB, rigid PCB, or both) to electrically connect to the CMUT. Further,the packaging substrate may also be used to seal a coupling medium withother components in the CMUT apparatus.

In some examples, the CMUT apparatus may include the capability tocontrol the shape or focus of the acoustic wave (or acoustic beam)output by the CMUT(s). This capability may be generally referred toherein as the focusing functionality or focusing capability. As oneexample, the shape or focus of the acoustic output may be controlled byshaping an active surface of the CMUT(s) into a desired curvature. Asanother example, the output may be focused by introducing a desiredphase pattern (phase delay) in the acoustic wave generated by one CMUT.In still other examples, the shape or focus of the acoustic output maybe controlled by an acoustic lens positioned in the front of theCMUT(s). In other examples, the some or all of the above techniques maybe combined.

Example CMUT Apparatuses

FIG. 1A illustrates a cross-sectional view of an example CMUT apparatus100 including an acoustic window according to some implementations. TheCMUT apparatus 100 includes one or more CMUTs 110, which may be a singleCMUT or multiple CMUTs, such as in a CMUT array. In this example, theCMUT 110 may include at least a first electrode 112 and a secondelectrode 114 positioned with respect to one another to form atransducing space 116 (i.e., an electrode separation gap between the twoelectrodes 112, 114). For example, the first electrode 112 may bemounted on a flexible membrane 118, a spring member, or the like, thatmay be displaced to change the transducing space 116.

As one example, in a transmission mode, an electric signal may cause achange in the transducing space 116 between the first electrode 112 andthe second electrode 114 to cause acoustic output to be directed towardan acoustic window 120 through a coupling medium 130. Alternatively, ina reception mode, acoustic energy may pass through the acoustic window120 and the coupling medium 130 to cause a change in the transducingspace 116, thereby producing a signal indicative of the receivedacoustic energy. Further, implementations herein are not limited to anyparticular CMUT configuration, electrode locations, type of springelements, flexible membrane arrangements, or the like. Additionalnon-limiting examples of suitable CMUTs and CMUT arrays that may be usedwith the acoustic windows and CMUT apparatuses herein are described inU.S. patent application Ser. No. 11/914,597, filed Feb. 12, 2009, andU.S. Pat. Nos. 7,564,172, 8,018,301, and 8,120,229, all to Yongli Huang,the entire disclosures of which are incorporated by reference herein.

In some examples, a protection layer (not shown in FIG. 1A) can belayered over an outer surface of the CMUT 110 to isolate the CMUT 110and the electrodes 112, 114 from the coupling medium 130. The protectionlayer may be used if the coupling medium 130 is not non-conductive(e.g., conductive or semiconductive). The protection layer may be anysuitable insulating or dielectric material, such as poly(p-xylylene)(e.g., Parylene®), polyimide, oxide, nitride, RTV, urethane,polyurethane, non-conductive polymer, or other suitable plastic orrubber materials.

The CMUT apparatus 100 may further include a CMUT packaging substrate140, which, in some examples, may at least partially support the CMUT110 and the coupling medium 130. For example, the CMUT 110 may bepackaged on the packaging substrate 140. The packaging substrate 140 mayprovide electrical connections (not shown in FIG. 1A) to the firstelectrode 112 and the second electrode 114, as well as providingstructural support to the CMUT 110 and the CMUT apparatus 100. Thepackaging substrate 140 may be of any suitable material, such asplastic, rubber, resin, metal, ceramic, or the like. An example of atypical packaging substrate for a CMUT includes at least a PCB (printedcircuit board).

The packaging substrate 140 may support a CMUT substrate 142 that may beof silicon, glass or other suitable material for fabricating the one ormore CMUTs 110. For example, one or more CMUT cells 150 of one or moreCMUTs 110 may be deposited or otherwise built or positioned on the CMUTsubstrate 142. In some cases, there may be a plurality of CMUT cells 150for each CMUT 110. Further, there may be a plurality of such CMUTs 110built on the substrate 142 to form a 1D, 1.5D, 1.75D or 2D array ofCMUTs.

In some implementations, each CMUT cell 150 may include a firstelectrode 112 and a second electrode 114, as shown; however, in otherimplementations, some or all of the CMUT cells 150 may share commonfirst and/or second electrodes (not shown in FIG. 1). Additionally, insome implementations, the first and second electrodes 112, 114 may becommonly addressed for the entire CMUT 110, i.e., they may share anelectrical connection, or may be a single common electrode that extendsacross all the cells 150. In other implementations, however, one or bothof the electrodes 112, 114 of each cell 150 may be individuallyaddressed. In some cases, the cells 150 may be of the same or similardesign; while in other cases, the cells 150 may be of two or moredifferent designs in the same CMUT 110. Further, in some examples, asdiscussed below with respect to FIG. 1B, each CMUT 110 in a CMUT arraymay be separately addressable, such that individual CMUTs 110 in anarray may be independently activated in transmission mode, or a signalmay be independently detected therefrom in reception mode.

Further, the acoustic window 120 may wholly or partially enclose thecoupling medium 130 to protect the one or more CMUTs 110 from directlycontacting an external medium (not shown in FIG. 1A). The acousticwindow 120 and the coupling medium 130 may also be configured orselected to effectively pass acoustic energy with minimal absorption ordistortion. The acoustic window 120 is shown in the example of FIG. 1Ahaving a random shape or thickness profile. Accordingly, in the exampleof FIGS. 1A and 1 n others of the following figures, except wherespecifically described otherwise, the acoustic window 120 may be anacoustic window of any suitable shape or thickness profile. For example,the acoustic window may be designed to have a desired shape andthickness profile to achieve desired mechanical and acoustic properties,as discussed additionally below.

In some examples, the acoustic window 120 may be solid material that isable to provide protection mechanically for the CMUT apparatus 100. Thethickness of the acoustic window 120 may be thin enough to have minimuminternal acoustic loss, but thick enough to protect the CMUT apparatus100 during use, such as when contacting an external medium. In someexamples, the acoustic window 120 may have a thickness in the range of0.1 to 20 wavelengths of the acoustic wave in the frequency at which theCMUT 110 operates. Accordingly, the acoustic window 120 may bestructurally strong enough to be pressed against an external medium,such as when the CMUT apparatus is part of a medical probe or otherinstrument.

For medical imaging applications, suitable materials for the acousticwindow 120 include, but are not limited to, plastics and rubbermaterial, such as polyethylene or polyurethane formulations,polymethylpentene, acrylonitrile butadiene styrene (ABS), polycarbonateABS (PC-ABS), thermoplastic polycarbonate (e.g., Makrolon®),polysulfone, cross-linked polystyrene microwave plastic (e.g.,Rexolite®), polyamides, and so forth. In some examples, the material ofthe acoustic window 120 may be chosen to have an acoustic property, suchas an acoustic impedance, that matches (i.e., the same, similar or closeto) the acoustic properties (e.g., acoustic impedance) of the intendedor targeted external medium (e.g. human tissue, which has acousticimpedance about 1.5 MRayl). For example, if the external medium is humantissue, the acoustic impedance of the acoustic window may be chosen inthe range of 1-4 MRayl. In more specific examples, the acousticimpedance of the acoustic window may be chosen in a range of 1.5+/−0.5MRayl. In other examples, such as for other applications of the CMUTapparatuses herein, a suitable material for the acoustic window 120 mayhave an acoustic impedance that closely matches that of other respectivetarget external media, such as within the same order of magnitude orless. Further, the acoustic impedance of the acoustic window 120 may bethe same as, or close to, that of the coupling medium 130 (e.g., in thesame ranges as discussed above) in addition to that of the target medium(e.g., human tissue, etc.) outside the CMUT apparatus 100. If theimpedance of the acoustic window has a relatively large mismatch withthe intended or targeted external medium or the coupling medium 130,then one or more additional (matching) layer(s) or transition layer(s)may be built on the acoustic window,

As discussed additionally below, the acoustic window 120 and/or thecoupling medium 130 in some examples may not need to have a focusingcapability. Therefore, more materials are available to be used for theacoustic window 120 and coupling medium 130 than if an acoustic lens isused as the acoustic window 120 with the CMUT apparatus 100.Additionally, in some examples, an acoustic lens (e.g., made of RTV ormade from other material listed above) can also be included with theacoustic window 120 on the CMUT apparatus 100.

The coupling medium 130 serves to couple acoustic energy, such as anacoustic wave, between the CMUT 110 and the acoustic window 120. Thecoupling medium 130 can be liquid-based, solid-based, or gel-based, etc.The coupling medium may have a low acoustic loss and an acousticimpedance that matches with that of CMUT 110 and/or the acoustic window120. Accordingly, the coupling medium 130 may be selected to have aminimum impact on both the acoustic energy transfer and the frequencyresponse of the CMUT apparatus 100. Furthermore, in some examples, ifthe coupling medium is solid-based, the coupling medium may have aYoung's modulus smaller than 5 GPa. In more specific examples, thecoupling medium may have a Young's modulus smaller than 1 GPa.

Additionally, in some examples, the coupling medium 130 may not benon-conductive material (e.g., the coupling medium 130 may beconductive). In these cases, a ground electrode (GND) of the CMUT (e.g.,the first electrode 112) may be designed to face and contact thecoupling medium 130, while a second electrode (e.g., second electrode114) may be insulated from the coupling medium 130. For instance, for aCMUT array that has a common electrode among multiple CMUT elements, thecommon electrode of the CMUT elements is typically also the groundelectrode (GND) of the CMUT elements.

Some example solid-based materials suitable for the coupling medium 130include, but are not limited to, plastics and rubber material,polydimethylsiloxane (PDMS), poly(p-xylylene) (e.g., Parylene®), RTV,silicone (e.g. PDMS), nitride, oxide, Riston® dry film photoresist,polyimide films (e.g., Kapton®), photoresist, polyimide, urethane,polyurethane, cross-linked polystyrene microwave plastic (e.g.,Rexolite®), polyethylene, other polymers, and so forth.

Additionally, materials used for the coupling medium 130 may beliquid-based, such as water-based, gel-based, oil-based, or syntheticpolymeric liquids, etc. Some example suitable liquid-based materials forthe coupling medium 130 include, but are not limited to, methylsalicylate, giycol, silicone gel, methylsilicone oil, mixtures of waterand glycol, and the like. In addition, the materials may be chosen fromthe group consisting of liquid, a gel, and a colloid. Examples ofliquids include, without limitation, water, a saline solution, glycerol,castor oil, mineral oil, vegetable oil and fluorocarbon-based fluid(e.g., Fluorinert®), and the like. Moreover, in some examples, thecoupling medium 130 may be a mixture of 1-Butanol in Glycerol. Forinstance, the attenuation of the mixture may be adjusted withoutimpairing acoustic velocity matching by adding a suitable amount of2-Hydroxyethyl ether.

Acoustic energy, such as an acoustic beam, may pass through the acousticwindow 120 and the coupling medium 130 in an operational direction 160to or from the one or more CMUTs 110. For example, in a transmissionmode, the CMUT 110 or the CMUT array may emit acoustic energy toward theacoustic window 120, generally in the operational direction 160.Similarly, in a reception mode, the CMUT 110 or the CMUT array mayreceive acoustic energy passing through the acoustic window 120 and thecoupling medium 130 in the operational direction 160.

The thickness of the coupling medium 130 may be thin enough to haveminimum internal acoustic loss, but thick enough to serve as a bufferlayer between the acoustic window 120 and the CMUT 110 so that theproperties of the selected acoustic window 120 has a minimum impact onthe performance of the CMUT 110. In some examples, the thickness of thecoupling medium 130 may be larger than ¼ of the wavelength of theacoustic wave in the frequency at which the CMUT operates.

FIG. 1B illustrates an example CMUT apparatus 170 including the acousticwindow 120 and a CMUT array 180 according to some implementations. Forinstance, in some cases, the CMUT apparatus may include a single CMUT110 while, in other cases, the CMUT apparatus may include multiple CMUTs110. In some examples, the multiple CMUTs 110 may be configured into anarray, such as the CMUT array 180, with each CMUT 110 serving as anelement of the CMUT array 180. In the illustrated example, the CMUTarray 180 includes a plurality of the CMUTs 110 described above withrespect to FIG. 1A. The CMUT array 180 may be any suitable arrayconfiguration, such as a 1D, 1.25D, 1.5D, 1.75D, 2D array, or the like.For example, the CMUT array 180 may have a linear pattern, atwo-dimensional array pattern, such as a grid pattern, a cross pattern,or any other desired arrangement. Thus, implementations are not limitedto any particular array configuration.

In the example of FIG. 1B, the substrate 142 may be conductive and,therefore, may serve as a common electrode for multiple CMUTs 110. Thefirst electrodes 112 may be individually addressable for each CMUT 110or, alternatively, may be individually addressable for each cell 150 ofeach CMUT 110. As another alternative, rather than having the substrate142 be conductive, the second electrodes 114 may be included in theCMUTs 110 in the example of FIG. 1B, similar to the example, describedabove with respect to FIG. 1A. Other variations will also be apparent tothose of skill in the art in light of the disclosure herein.

In the illustrated example, the CMUT array 180 is formed on a flat CMUTsubstrate 142 and a flat packaging substrate 140. Thus, the CMUTs 110 ofthe CMUT array 180 have active surfaces 182 facing in the operationaldirection 160, and may operate in the operational direction 160 totransmit and/or receive acoustic energy through the acoustic window 120and coupling medium 130. Thus, the active surface 182 of a CMUT 110 mayserve to receive acoustic energy in receiving mode or generate acousticenergy in a transmission mode. In other examples, the CMUT substrate 142and/or the packaging substrate 140 may be curved to form the curvedactive surface for a CMUT 110, thus to have focus capability. Forexample, a concave curve may at least in part focus each CMUT 110 in theCMUT array 180 toward a particular focal location, while a convex curvemay at least in part focus the CMUT 110 in a disparate direction.

FIG. 2 illustrates a cross-sectional view of an example CMUT apparatus200 having an acoustic window according to some implementations. TheCMUT apparatus 200 includes one or more CMUTs 210 (which may include oneor more CMUT arrays) with focusing capability. The CMUT apparatus 200further includes the acoustic window 120, the coupling medium 130, andthe CMUT packaging substrate 140. The CMUT 210 may include at least twoelectrodes, such as a first electrode 212 and a second electrode 214separated by a transducing space 216, which operate similarly to theCMUT 110 discussed above. For instance, the CMUT 210 may include aplurality of CMUT cells 250, each of which may include a first electrode212 and a second electrode 214.

Furthermore, in some implementations, the CMUT 210 may be part of a CMUTarray including a plurality of CMUTs 210, similar to the exampledescribed above with respect to FIG. 1B. In the case in which the CMUT210 is included in a CMUT array, the CMUT 210 may include individualelectrodes, such as the first and second electrodes 212, 214, or mayinclude one or more common electrodes shared with one or more otherCMUTs 210 in the CMUT array. For example, the CMUTs 210 in a CMUT arraymay share one common electrode as a first electrode and have individualindependently addressable electrodes as a second electrode, as discussedabove with respect to FIG. 1B, and as discussed additionally below.

In some cases, the CMUT apparatus 200 may include a focusing capabilityto shape the acoustic output of the CMUT 210. For example, if theacoustic window 120 and the coupling medium 130 do not have a focusingfunctionality, the CMUT 210 may be designed to have focusing capabilityto focus the acoustic energy at a focal area or focal location 270. Forinstance, the CMUT 210, which may include a CMUT substrate 242, may beformed or shaped such that an active surface 282 of the CMUT 210 has adesired shape or curvature 272 to achieve a desired focus functiongenerally in the operational direction 160 for focusing the acousticenergy at the focal location 270. Additionally, in other examples, anacoustic lens that also includes focusing capability may also beincorporated into at least one of the acoustic window 120 or thecoupling medium 130 to provide additional focusing capability.

Alternatively, a CMUT or CMUTs in a CMUT array (i.e., including eitherthe CMUT 110 having a generally flat configuration, or a CMUT 210 havinga curved configuration) may be configured or operated to have anacoustic output with a desired non-uniform phase pattern, amplitudepattern, or both. For example, the flat or curved CMUT 110, 210,respectively, discussed herein, or the CMUTs 110, 210 configured in CMUTarrays, may be operated to focus on a desired focal location 270. Thus,by introducing a phase difference (e.g., a 180 degree phase difference)into the distribution for the acoustic wave emitted by various CMUTcells 150, 250 at different locations in a CMUT 110, 210, the acousticwave intensity can be enhanced at one or more particular focal areas orfocal locations 270, even though the CMUT itself is flat (CMUT 110) orof a fixed curvature (CMUT 210), Accordingly, by controlling theoperation phase and/or amplitude distribution of individual CMUT cellsin a CMUT or individual CMUTs in a CMUT array, the ultrasonic energy maybe focused to a certain extent even though the acoustic source is flator of a fixed curvature.

In the following examples, except when specifically mentioned, the CMUTsand/or CMUT arrays in the CMUT apparatuses described in the followingimplementations, may or may not have focusing capabilities. In addition,while a single CMUT 110 or 210 may be shown in the examples, it is to beunderstood that the CMUT 110, 210 in the examples, in some cases, may beonly one element of a larger CMUT array including a plurality of CMUTs110 and/or 210. Further, other types of CMUTs may be used in theexamples herein or intermixed with one another in the arrays herein inaddition to, or as an alternative to, the CMUTs 110, 210 described inthe examples herein.

FIG. 3 illustrates a cross-sectional view of an example of a CMUTapparatus 300 with the acoustic window 120 and a sealed coupling medium130. For example, in the situation that the coupling medium 130 is aliquid-based or gel-based material, the coupling medium 130 may beenclosed in a sealed space. Accordingly, a case or housing 302 may beused to seal the coupling medium 130 with the acoustic window 120, CMUTpackaging substrate 140, the CMUT 110 or 210 and any other components.

In the example, of FIG. 3, the housing 302 may enclose and contain orretain the coupling medium 130 and the CMUT 110. A first seal 304 may beformed between the housing 302 and the packaging substrate 140, and asecond seal 306 may be formed between the housing 302 and the acousticwindow 120. The seals 304, 306 may be formed using any suitabletechnology, such as adhesion material (e.g., epoxy, glue, etc.),O-rings, molding, (thermal) compression, interference fit, shrink fit,bonding without adhesive, magnetic attraction, electro-staticattraction, or any other suitable sealing technique. The sealing can bealso engineered to provide acoustic decoupling between differentcomponents at the seal location.

FIG. 4 illustrates a cross-sectional view of an example CMUT apparatus400 with an acoustic window 420 and sealed coupling medium 130 accordingto some implementations. In this example, the material of the acousticwindow 420 itself may be used to contain the coupling medium 130. Forinstance, the acoustic window 420 may be formed to include a front orwindow portion 422 and sidewall(s) 424. Accordingly, the acoustic window420 may also serve as a housing for enclosing and containing thecoupling medium 130 in contact with the other components such as theCMUT 110 or 210 and the packaging substrate 140.

In the illustrated example, the CMUT apparatus 400 includes the CMUT210, the acoustic window 420, the coupling medium 130, and the CMUTpackaging substrate 140. The acoustic window 420 serves at least twofunctions: (1) an acoustic window function, which is served by thewindow portion 422, and (2) a packaging function, which is served by thesidewall(s) 424. Thus, the window portion 422 is used to pass acousticenergy to or from the CMUT 210 in the operational direction 160. Thesidewalls 424 are not used to pass usable acoustic energy, but insteadserve to enclose or seal the coupling medium 130 and retain the couplingmedium 130 with the other components, such as the CMUT 210 and thesubstrate 140. Thus, a seal 426 may be formed between the sidewall(s)424 and the packaging substrate 140. Compared with the implementation300 discussed above with respect to FIG. 3, one of the sealinginterfaces, i.e., second seal 306 may be eliminated during the assemblyprocess in the implementation of FIG. 4. The seal 426 may be formedusing any suitable technique, such as those discussed above with respectto FIG. 3.

FIGS. 5-20 provide several examples of acoustic windows and couplingmedia according to some implementations. For clarity of illustration, ahousing or other enclosure or retaining structure is not shown in FIGS.5-20. However, any of the sealing and retaining techniques describedabove such as those described in FIGS. 3-4, or others, may be used inany of the implementations herein.

FIG. 5 illustrates a cross-sectional view of an example of a CMUTapparatus 500 with an acoustic window 520 according to someimplementations. The CMUT apparatus 500 may include a CMUT (or a CMUTarray), such as the CMUTs 110 or 210 discussed above. In the illustratedexample, the CMUT apparatus 500 includes the CMUT 210, the acousticwindow 520, the coupling medium 130, and the CMUT packaging substrate140. The acoustic window 520 may have a flat or planar configurationwith a uniform thickness profile in cross section. In order to minimizeacoustic reflection (e.g., at a transition or interface 522 between thecoupling medium 130 and the acoustic window 520, or at the interface 524between the acoustic window 520 and an external medium (e.g., air,tissue, etc.), the acoustic impedances of the acoustic window 520 andthe coupling medium 130 may be selected to be close to the impedance ofthe target medium (e.g., human tissue in some examples).

In some cases, when there is an acoustic impedance mismatch among theacoustic window 520, the coupling medium 130 and/or the target medium,the thickness of both the acoustic window 520 and the coupling medium130 may be selected to minimize the acoustic reflection from theacoustic window 520. One example technique to minimize acousticreflection is to have the reflected waves from the different interfaces522, 524 at least partially cancel out each other. For example, in thecase of an impedance mismatch among the acoustic window 520, thecoupling medium 130 and the target medium, the thickness of the acousticwindow 520 or the coupling medium 130, or both, may be ¼ or ½ of theacoustic wavelength, or multiples of ¼ or ½ of the acoustic wavelength,of the acoustic frequency at which the CMUT 210 operates. This thicknessselection can potentially minimize the acoustic reflection. Further,since the flat acoustic window 520 does not provide a focusingcapability (or lens function), the CMUT 210 may provide a focusingcapability, as discussed above, if it is desired to focus the CMUTapparatus 500 on a particular focal location or focal area.

FIG. 6 illustrates a cross-sectional view of an example CMUT apparatus600 with an acoustic window 620 according to some implementations. TheCMUT apparatus 600 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 600 includes the CMUT 210, the acoustic window 620, thecoupling medium 130, and the CMUT packaging substrate 140. The acousticwindow 620 may have an overall convex shape facing outward from the CMUT210 and a uniform thickness profile. In this example, there may be lessreflection from an interface 622 of acoustic window 620 with thecoupling medium 130 and/or an interface 624 of the acoustic window 620with the target medium than in the implementation of FIG. 5. However,both the acoustic impedances and the thicknesses of the acoustic window620 and the coupling medium 130 may be selected the same way as thatdescribed for the CMUT apparatus 500 having the flat acoustic window 520illustrated in FIG. 5.

Further, in this example, in the case that the velocity of the acousticwave or acoustic energy in the coupling medium 130 is different from(e.g., slower than) that in the outside target medium, the couplingmedium 130 may provide a focusing capability and may serve as a de factoacoustic lens. Therefore, in some examples, the CMUT used might not havea focusing capability. For example, the CMUT 110 or other non-focusedtransducer may be used, rather than a focused transducer, such as theCMUT 210.

FIG. 7 illustrates a cross-sectional view of an example CMUT apparatus700 with an acoustic window 720 according to some implementations. TheCMUT apparatus 700 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 700 includes the CMUT 210, the acoustic window 720, thecoupling medium 130, and the CMUT packaging substrate 140. The acousticwindow 720 has a concave shape facing outward from the CMUT 210 in theoperational direction 160, and a uniform thickness profile. In thisexample, there may be less reflection from and interface 722 of theacoustic window 720 with the target medium and/or an interface 724 ofthe acoustic window 702 with the coupling medium 130 than in theimplementation of FIG. 5. However, both the acoustic impedances and thethicknesses of the acoustic window 720 and the coupling medium 130 maybe selected the same way as that described above for the CMUT apparatus500 having the flat acoustic window 520 shown in FIG. 5.

Further, in the case that the acoustic velocity of the acoustic wave inthe coupling medium 130 is different from (e.g., faster than) that inthe external target medium, the coupling medium 130 may have a de factofocusing capability and may serve as a lens. Therefore, the CMUT usedmight not have a focusing capability. For example, the CMUT 110 may beused in the CMUT apparatus 700.

FIG. 8 illustrates a cross-sectional view of an example CMUT apparatus800 with an acoustic window 820 according to some implementations. TheCMUT apparatus 800 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 800 includes the CMUT 210, the acoustic window 820, thecoupling medium 130, and the CMUT packaging substrate 140. The acousticwindow 820 may have a flat outer face 822, a flat inner face 824, andmay have a uniform thickness. Further, the acoustic window 820 may forma plane that is tilted at an angle θ with respect to the CMUT 210 andthe operational direction 160. For example, the outer face 822, theinner face 824, or another portion of the acoustic window 820 maygenerally form an overall plane or a major plane that is tilted at theangle θ with respect to the operational direction 160 of the CMUT 210.For example, the operational direction 160 may generally include thedirection in which the one or more CMUTs 210 emit acoustic energy and/orthe direction from which the one of more CMUTs 210 are positioned toeffectively receive acoustic energy. Also, FIG. 8 shows that theacoustic window 820 is tilted with respect to the CMUT 210; however, thetilting between the CMUT 210 and the acoustic window 820 may be relativeto one another in FIG. 8, e.g. the configuration have the same effect ifthe CMUT 210 is tilted with respect to the acoustic window 820.

Accordingly, acoustic energy, such as an acoustic beam that passesthrough the acoustic window 820 will pass through in the operationaldirection 160 at a non-perpendicular or oblique angle based at least inpart on the angle of tilt θ of the plane of the acoustic window 820. Forexample, in transmission mode, the one or more CMUTs 210 may emitacoustic energy toward the acoustic window 820, generally in theoperational direction 160. Similarly, in reception mode, the one or moreCMUTs 210 may receive acoustic energy passing through the acousticwindow 820 in the operational direction 160. Accordingly, thepositioning of the acoustic window 820 at the angle θ with respect tothe operational direction 160 of travel of the acoustic energy canfurther reduce the reflection from the acoustic window 820.

The tilt angle θ may be selected to direct any reflected acoustic energyaway from the CMUT 210, and may have a minimum impact on the shape anddirection of the acoustic energy that passes through the acoustic window820. Moreover, both the acoustic impedances and the thicknesses of theacoustic window 820 and the coupling medium 130 may be selected in amanner similar to that described above with respect to the CMUTapparatus 500 with the flat acoustic window 520 of FIG. 5. Additionally,since the flat acoustic window 820 does not have a focusing capability(or a lens function), the CMUT 210 may include a focusing capability ifit is desired to focus the CMUT apparatus 800.

If the acoustic transmission properties (e.g., acoustic velocity oracoustic impedance) of the coupling medium 130 and/or that of an outsidetarget medium 828 are substantially different from that of the acousticwindow 820 at the operation frequency of the CMUT 210, then the acousticbeam may be steered or refracted away from its original direction duringpassage through an interface between the inner face 824 and the couplingmedium 830 and/or during passage through an interface between the outerface 822 and the outside target medium 828 (e.g., human tissue in thecase of a medical probe). If this refractive effect is desired to beavoided, an alternative configuration of the acoustic window may beused, such as the configurations of FIG. 9 or 10.

FIG. 9 illustrates a cross-sectional view of an example of a CMUTapparatus 900 with an acoustic window 920 according to someimplementations. The CMUT apparatus 900 may include a CMUT (or a CMUTarray), such as the CMUTs 110 or 210 discussed above. In the illustratedexample, the CMUT apparatus 900 includes the CMUT 210, the acousticwindow 920, the coupling medium 130, and the CMUT packaging substrate140. The acoustic window 920 may include a uniform thickness profile;however, instead of tilting a flat-plate or planar acoustic window, asdescribed above with respect to FIG. 8, in this implementation, theacoustic window 920 may include a plurality of tilted or angled facets922. The angle Φ of tilt of the facets 922 may be selected to reduce thereflection from the acoustic window 920 to the CMUT 210. The geometricsize of the facets 922 may be designed in a range of 0.1 times to 10times of the acoustic wavelength at which the transducer operates. Forexample, the range of the height H of the facets 922 may be selected tobe in a range of 0.1 times to 1 times of the acoustic wavelengthgenerated by the CMUT 210. Further, the range of the length L of thefacets 922 may be selected to be in a range of 0.25 times to 10 times ofthe acoustic wavelength generated by the CMUT 210.

In addition, FIG. 9 illustrates a cross section of the window 920,showing the facets in a first direction. In some examples, the facets922 may be longitudinal parallel facets that tilt in alternate directionby the angle Φ. In other examples, the facets 922 may be formed in twodirections, such as in the shape of a plurality of adjacent pyramids.Additionally, in the illustrated example, the facets 922 may be designedas the periodic pattern of alternating angles of tilt, as shown in FIG.9. Alternatively, the facets 922 may be designed as non-periodicpatterns with different heights, lengths, and/or angles of tilt amongthe plurality of facets 922.

FIG. 10 illustrates a cross-sectional view of an example CMUT apparatus1000 with an acoustic window 1020 according to some implementations. TheCMUT apparatus 1000 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1000 includes CMUT 210, the acoustic window 1020, the couplingmedium 130, and the CMUT packaging substrate 140. The acoustic window1020 may have a uniform thickness profile. Further, rather than havingmany facets with tilted flat surfaces as illustrated in FIG. 9, theacoustic window 1000 may be undulated into continuously curved patterns1022 to effectively reduce the reflection to the CMUT 210 from theacoustic window 1020. For example, the curved or undulating patterns1022 may be formed as a sine wave or other gradually changing shapeshaving a series of peaks 1024 and valleys 1026 with respect to at leastone direction. The geometric size of the patterns 1022 may be selectedto be in a range of 0.1 times to 10 times of the acoustic wavelength atwhich the CMUT 210 operates. For example, the range of the height H ofthe patterns 1022 may be selected to be in a range of 0.1 times to 1times of the acoustic wavelength that the CMUT 210 generates. Further,the range of the length L (e.g., from one valley to the next valley orfrom one peak to the next peak) of the patterns 1022 may be selected tobe in a range of 0.25 times to 10 times of the acoustic wavelengthgenerated by the CMUT 210. The patterns 1022 of the acoustic window canbe designed as regular or periodic patterns as shown in FIG. 10. In somecases, the pattern 1022 of undulations may be formed in one direction asa plurality of generally parallel elongated peaks 1024 and channels orvalleys 1026, giving the acoustic window 1020 a corrugated surfacepattern. In other examples, the pattern of undulations may be formed intwo directions providing a plurality of individual peaks 1024 surroundedby valleys 1026 on all sides. As still another example, the patterns1022 may be designed as non-periodic patterns with different heightsand/or lengths. As still another example, the non-periodic pattern mayextend in only one direction or may extend in two directions across theacoustic window 1020.

Furthermore, rather than the acoustic windows having a uniform thicknessprofile, such as are illustrated in FIGS. 5-10, the acoustic windowsherein may have a non-uniform thickness profile or cross sectionalthickness to achieve one or more desired acoustic effects. FIGS. 11-17illustrate some example implementations of acoustic windows having anon-uniform cross sectional thickness, such as having at least twoportions with different thicknesses.

FIG. 11 illustrates a cross-sectional view of an example CMUT apparatus1100 with an acoustic window 1120 according to some implementations. TheCMUT apparatus 1100 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1100 includes the CMUT 210, the acoustic window 1120, couplingmedium 130, and CMUT packaging substrate 140. The thickness profilepattern of the acoustic window 1120 may be designed as a thicknessprofile having multiple steps. FIG. 11 shows an example acoustic window1120 having a pattern 1122 that that includes two relatively definedthicknesses, i.e., a first thickness 1124 and a second thickness 1126,alternating at regular intervals. Thus, the pattern 1122 may be asurface pattern formed into at least one surface of the acoustic window1120.

The geometric size of the thickness profile pattern 1122 may be in arange of 0.1 times to 10 times of the acoustic wavelength generated bythe CMUT 210. For example, the range of a height D of the pattern 1122may be in a range of 0.1 times to 1 times of the acoustic wavelengthgenerated by the CMUT 210; and the range of a length L of the pattern1122 may be designed in a range of 0.25 times to 10 times of theacoustic wavelength generated by the CMUT 210. In addition, the pattern1122 of the acoustic window 1120 may be implemented as a plurality ofperiodic surface patterns 1122 as shown in FIG. 11. In some cases, thepattern 1122 may extend in one direction providing a plurality ofelongated portions of different heights D. In other examples, thepattern 1122 may extend in two directions providing a checkerboard typearrangement of portions of different heights D. The pattern 1122 mayalternatively be a non-periodic pattern having differing heights D andlengths L. In addition, an uneven surface 1128 of the pattern 1122 ofthe acoustic window 1120 illustrated in FIG. 11 faces the couplingmedium 130 and the CMUT 210. In other examples, the uneven surface 1128of the pattern 1122 may face the outside target medium (e.g. humantissue, etc.).

FIG. 12 illustrates a cross-sectional view of an example CMUT apparatus1200 with an acoustic window 1220 according to some implementations. TheCMUT apparatus 1200 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1200 includes the CMUT 210, the acoustic window 1220, thecoupling medium 130, and the CMUT packaging substrate 140. The thicknessprofile pattern 1222 of the acoustic window 1220 may be designed as athickness profile with multiple steps. In this example, uneven surfacepatterns 1222 may be formed on both sides or surfaces of the acousticwindow 1220, as shown in FIG. 12. Two uneven patterns 1222 may bealigned to provide a first thickness 1224 that alternates with a secondthickness 1226 in a manner similar to that discussed above with respectto FIG. 11.

The geometric size of the thickness profile pattern 1222 may be designedin a range of 0.1 times to 10 times of the acoustic wavelength generatedby the CMUT 210. For example, the range of a height D of the pattern1222 may be in a range of 0.1 times to 1 times of the acousticwavelength generated by the CMUT 210; and the range of a length L of thepattern 1222 may be designed in a range of 0.25 times to 10 times of theacoustic wavelength generated by the CMUT 210. In addition, the pattern1222 of the acoustic window 1220 may be implemented as a plurality ofperiodic patterns 1222 as shown in FIG. 12. In some cases, the pattern1222 may extend in one direction providing a plurality of elongatedportions of different heights D on two sides of the acoustic window1220. In other examples, the pattern 1222 may extend in two directionsproviding a checkerboard type arrangement of portions of differentheights D on two sides of the acoustic window 1220. The pattern 1222 mayalternatively be a non-periodic pattern having differing heights D andlengths L.

FIG. 13 illustrates a cross-sectional view of an example CMUT apparatus1300 with an acoustic window 1320 according to some implementations. TheCMUT apparatus 1300 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1300 includes the CMUT 210, the acoustic window 1320, thecoupling medium 130, and the CMUT packaging substrate 140. In theexample, of FIG. 13, instead of the multiple-step profiles shown inFIGS. 11-12, the thickness profile of the acoustic window 1320 maygradually change, as illustrated in FIG. 13 in a pattern 1322 having athin portion 1324 and a thick portion 1326. The geometric size of thethickness profile pattern 1322, shown in FIG. 13, may be in a range of0.1 times to 10 times of the acoustic wavelength of the acoustic wavegenerated by the CMUT 210. For example, the range of a height D ofpattern 1322 may be designed in a range of 0.1 times to 1 times of theacoustic wavelength generated by the CMUT 210; and the range of a lengthL of the pattern 1322 may be designed in a range of 0.25 times to 10times of the acoustic wavelength generated by the CMUT 210. In addition,the pattern 1322 may be formed on both sides of the acoustic window 1320or only on one side, and the pattern 1322 may be formed in onedirection, or in two directions. Alternatively, the pattern 1322 may bealso designed as a non-periodic pattern with different heights D andlengths L.

In FIGS. 11-13, the acoustic windows 1120, 1220, 1320 having non-uniformthickness profiles are illustrated using an example implementation witha flat acoustic window (e.g., related to the flat shape of the acousticwindow 520 shown in FIG. 5). In other examples, the acoustic windows1120, 1220, 1320 having a non-uniform thickness profile shown in FIGS.11-13 may be also implemented for the acoustic windows with differentshapes or positions. For example, the acoustic windows 1120, 1220, and1320 may be provided with the tilted configuration of FIG. 8 and/or theconvex or concave shapes of the acoustic windows shown in FIGS. 6-7.

As an alternative to the examples shown in FIGS. 11-13, any other typeof surface roughness, pattern of indentations, or other surface patternmay be generated on the surface of the acoustic windows for use with theCMUT apparatuses herein. The geometric size of the pattern may be in arange of 0.1 times to 10 times of the acoustic wavelength of theacoustic wave generated by the CMUT 110, 210. For example, the range ofa height of the pattern may be designed in a range of 0.1 times to 1times of the acoustic wavelength generated by the CMUT 110, 210; and therange of a lateral size of the pattern indentations, etc., may bedesigned in a range of 0.25 times to 10 times of the acoustic wavelengthgenerated by the CMUT 110, 210.

FIG. 14 illustrates a cross-sectional view of an example CMUT apparatus1400 with an acoustic window 1420 according to some implementations. TheCMUT apparatus 1400 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1400 includes the CMUT 110, the acoustic window 1420, thecoupling medium 130, and the CMUT packaging substrate 140. FIG. 14illustrates an example implementation of a CMUT apparatus 1400 with theacoustic window 1420 having a focusing capability. The thickness profileof the acoustic window 1420 forms a concave lens. In this case, an outersurface 1422 of the acoustic window 1420 include a concave curvature towhile an inner surface 1424 of the acoustic window 1420 adjacent to thecoupling medium 130 may be flat so that the coupling medium does notcontribute to the focus capability. Thus, in this example, there aremore selections available for the material of the coupling medium 130since the acoustic velocity or impedance of the coupling medium 130 isnot a factor. Further, in any of the examples of FIGS. 14-20, the CMUT210 may be used instead of the CMUT 110 to provide further focusingcapability.

FIG. 15 illustrates a cross-sectional view of an example CMUT apparatus1500 with an acoustic window 1520 according to some implementations. TheCMUT apparatus 1500 may include a CMUT (or a CMUT array), such as theCMUTs 110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1500 includes the CMUT 110, the acoustic window 1520, thecoupling medium 130, and the CMUT packaging substrate 140. In thisexample, the coupling medium 130 may also serve, at least in part, as anacoustic lens and may form a compound lens with the acoustic window1520. The acoustic window 1520 may include an outer surface 1522 havinga first curvature, and an inner surface 1524 of the acoustic window 1520may have a second curvature, different from the first curvature.Depending on the velocity of the coupling medium 130, the secondcurvature of the inside surface of the acoustic window may be designedto be either more concave than the first curvature, as shown in FIG. 15.In such an arrangement, the coupling medium 130 may contribute to thefocusing capability of the CMUT apparatus 1500. Additionally, in otherexamples, the second curvature may be less concave than the firstcurvature, or may alternatively be convex.

FIG. 16 illustrates a cross-sectional elevation view of an example CMUTapparatus 1600 with an acoustic window 1620 according to someimplementations. The CMUT apparatus 1600 may include a CMUT (or a CMUTarray), such as the CMUTs 110 or 210 discussed above. In the illustratedexample, the CMUT apparatus 1600 includes the CMUT 110, the acousticwindow 1620, the coupling medium 130, and the CMUT packaging substrate140. FIG. 16 shows another example implementation of a CMUT apparatuswith the acoustic window 1620 having a focusing capability. Theapparatus 1600 includes a thickness profile in which an outer surface1622 of the acoustic window 1620 forms a convex lens. In this case, aninner surface 1624 of the acoustic window 1620 facing the couplingmedium 130 may be flat so that the coupling medium 130 does notcontribute to the focusing capability. In such an arrangement, there aremore selections available for the material of the coupling medium 130since the acoustic velocity or impedance of the coupling medium 130 isnot a factor.

FIG. 17 illustrates a cross-sectional elevation view of an example CMUTapparatus 1700 with an acoustic window 1720 according to someimplementations. The CMUT apparatus 1700 may include a CMUT (or a CMUTarray), such as the CMUTs 110 or 210 discussed above. In the illustratedexample, the CMUT apparatus 1700 includes the CMUT 110, the acousticwindow 1720, the coupling medium 130, and the CMUT packaging substrate140. In this example, the coupling medium 130 may serve, at least inpart, as an acoustic lens by forming a compound lens in conjunction withthe acoustic window 1720. In this example, an inner surface 1722 of theacoustic window 1720 may have a desired curvature and an outer surface1724 may be flat or may have a curvature also. Depending on the velocityof the coupling medium 130, the curvature of the inside surface 1722 ofthe acoustic window 1720 may be designed to be either convex as shown inFIG. 17 or concave, as discussed above with respect to FIG. 15.

In the example implementations shown in FIGS. 14-17, the uneven patterns(e.g., those patterns discussed above with respect to FIGS. 11-13) maybe formed on either the inner or outer surface, or both surfaces, of theacoustic windows 1420-1720 to weaken the effects of acoustic reflectionon the CMUT operation.

FIG. 18 illustrates a cross-sectional elevation view of an example CMUTapparatus 1800 with an acoustic window 1820 according to someimplementations. The CMUT apparatus 1800 may include a CMUT (or a CMUTarray), such as the CMUTs 110 or 210 discussed above. In the illustratedexample, the CMUT apparatus 1800 includes the CMUT 110, the acousticwindow 1820, the coupling medium 130, and the CMUT packaging substrate140. FIG. 18 illustrates an example in which the acoustic window 1820may be made of multiple layers. For example, by selecting propermaterials and layer thicknesses, the acoustic window 1820 may havedesired acoustic and/or mechanical properties. For instance, by usingmultiple layers to build the acoustic window 1820, an anti-reflectioncoating layer 1822 may be formed on the acoustic window 1820. Thecoating layer 1822 may be deposited on either an outer surface 1824 oran inner surface 1826, or both surfaces 1824, 1826 of the acousticwindow 1820.

In some examples, as illustrated, a different coating layer 1828 may beformed on the inner surface 1826, which is different from the coatinglayer 1822 formed on the outer surface 1824. As an example, if theacoustic impedance does not match between the acoustic window material1830 and the target medium outside the CMUT apparatus 1800, a matchinglayer 1822 may be added. Further, if the acoustic impedance does notmatch between the acoustic window material 1830 and the coupling medium130, a matching layer 1828 may be added. For example, the matchinglayers 1822, 1828 may have an acoustic impedance between the twounmatched mediums and the thickness of the matching layer may be ¼ or ½of the acoustic wavelength generated by the CMUT 110 (or ¼ and ½ plus anadditional full wavelength). The matching layers 1822, 1828 may be addedon any implementation of the acoustic windows in this disclosure whenthe acoustic impedances do not match between the acoustic window and theother mediums. Thus, the acoustic window 1820 made of multiple layers ofmaterial may be used for any acoustic window, including, but not limitedto, any of the implementations disclosed herein.

FIGS. 19A-C illustrate examples of a CMUT apparatus 1900 with anacoustic window 1920 according to some implementations. The CMUTapparatus 1900 may include a CMUT (or a CMUT array), such as the CMUTs110 or 210 discussed above. In the illustrated example, the CMUTapparatus 1900 includes the CMUT 110, the acoustic window 1920, thecoupling medium 130, and the CMUT packaging substrate 140. Moreover,instead of (or in addition to) coating a layer of selected material onthe acoustic window as discussed above with respect to FIG. 18, in theexample of FIG. 19A, at least a portion of the acoustic window 1920 mayinclude a surface layer 1922 engineered to have desired materialproperties on at least one of an outer surface 1924 or an inner surface1926.

FIG. 19B illustrates an enlarged view of an area of a surface layer 1928at the inner surface 1926 from FIG. 19A, including an example structuredsurface layer 1930 of the acoustic window 1920 according to someimplementations. The structured surface layer 1930 may include aplurality of trenches (or openings) 1932 formed into the surface layer1930 of the material of the acoustic window 1920. The trenches (oropenings) 1932 may have a height h, with a trench width w1 and a trenchpitch w2, The trenches 1932 may be formed on either or both of the innersurface 1926 or the outer surface 1924 of the acoustic window 1920. Inthe example of FIG. 19B, the trenches 1932 may be filled with thematerial of the coupling medium 130 or an exterior medium, such as airor acoustic gel, in the case that the structured surface layer 1930 isformed on the outer surface 1922 of the acoustic window 1920. Thus, thestructured surface layer 1930 has the acoustic properties different fromboth those of acoustic window material and the material filled into thetrenches of the structured surface layer 1930. In some cases, thetrenches 1932 may be formed in a single direction as a plurality of longparallel trenches. In other examples, the trenches 1932 may be formed intwo directions, such as perpendicular to one another, to form a grid oftrenches 1932.

Alternatively, as illustrated in FIG. 19C, the trenches 1932 may befilled with a filling material 1934. The acoustic properties of thesurface layer 1930 may be defined by the acoustic properties of both theacoustic window material 1936 and the filling material 1934, as well asthe geometry of the trenches (i.e., the height h, width w1, width w2,and patterns). Therefore, in some examples, the surface layer 1930 maybe engineered to have desired acoustic properties. Either or bothsurfaces of the acoustic window 1920 may be engineered to have thesurface layer 1930 with the desired properties. Thus, a suitable fillingmaterial 1934 having desired properties may be the same or similar tothe coupling medium 130 and/or the external or target medium. Typically,according to the material used for the acoustic window, the fillingmaterial 1934 may be the same or similar to those example materialsdisclosed for the coupling medium and the acoustic window discussedabove. In some examples, the filling material 1934 may have an acousticimpedance between the acoustic impedance of a window material 2034 andthe acoustic impedance of the coupling medium 130 when used on thesurface 1926 facing the coupling medium 130. The geometric size of thetrenches 1932 may be designed in a range of 0.1 times to 10 times of theacoustic wavelength of the acoustic wave at which the CMUT 110 operates.For example, the range of the height h, the width w1 and the width w2 ofthe trenches 1932 may be designed to be less than 1.0 acousticwavelength of the acoustic wave at which the CMUT operates.Particularly, the height h of the trenches 1932 may be designed to be ¼or ½ of the acoustic wavelength generated by the CMUT 110 (or ¼ and ½with an additional full wavelength).

FIGS. 20A-C illustrate an example CMUT apparatus 2000 with an acousticwindow 2020 according to some implementations. The CMUT apparatus 2000may include a CMUT (or a CMUT array), such as the CMUTs 110 or 210discussed above. In the illustrated example, the CMUT apparatus 2000includes the CMUT 110, the acoustic window 2020, the coupling medium130, and the CMUT packaging substrate 140. Moreover, the acousticproperties of the acoustic window 2020 may be controlled according to aselected pattern for trenches formed into at least one of an outersurface 2022 or an inner surface 2024 of the acoustic window 2020.

For example, as illustrated in FIG. 20B, an enlarge portion 2026 of theacoustic window 2020 includes a structured pattern 2028 including aplurality of trenches (or openings) 2030. The acoustic properties of theacoustic window 2020 may be controlled based at least in part on how farthe trenches 2030 extend, such as whether the trenches 2030 extendthrough a portion of the thickness of the acoustic window 2020 orentirely through. For example, the trenches may extend through anyportion of the thickness of the acoustic window 2020 to a height h.Further, in some cases not all trenches will extend the same height h.In addition, the trenches may have a width w1 and a pitch w2, similar tothe example described with respect to FIGS. 19A-C. In the example ofFIG. 20, the trenches 2030 are filled with a filling material 2032. Insome examples, the engineered transition layer on the acoustic window2020 with the filling material 2032 may have an acoustic impedancebetween the acoustic impedance of a window material 2034 and acousticimpedance of the filling material 2032 or the coupling medium 130, suchas when the trenches open to the side of the acoustic window facing thecoupling medium.

Alternatively, as illustrated in FIG. 20C, the trenches (or openings)2030 may extend entirely through the thickness of the acoustic window2020. In some cases, the trenches may be holes, while in other cases,the trenches may be longitudinal channels formed in one or multipledirections, at least some of which are generally parallel to oneanother. The filling material 2032 may be filled into the trenches 2030.The properties of the filling material 2032 may be selected to achieve adesired overall acoustic performance, with the material of the acousticwindow 2034 also being taken into consideration. The ranges of the widthw1 of the window material 2034 and the width w2 of the trenches 2030 maybe designed to be less than 1.0 of the acoustic wavelength at which thetransducer operates. The structured surfaces and windows may be used foracoustic windows with any shape or thickness profile including any ofthose disclosed herein.

FIG. 21 illustrates a perspective view of an example 1D CMUT arrayapparatus 2100 having an acoustic window 2120 according to someimplementations. In the illustrated example, the acoustic window 2120may correspond to the acoustic window 1020 described above with respectto FIG. 10, but any of the other acoustic windows, or combinationsthereof, described herein may be used for the acoustic window 2120. Fora 1D CMUT array, the acoustic effect of the acoustic window 2120 on eachtransducer element may be substantially the same. Accordingly, inexamples that include a 1D array, FIGS. 1-20 may provide example crosssection elevation views of the CMUT apparatuses along an elevationdirection 2122 of 1D CMUT array as viewed along line 21-21 indicated inFIG. 21 (and rotated 90 degrees counterclockwise).

FIG. 21 shows an example perspective view of a 1D CMUT array apparatus2100. The 1D CMUT apparatus includes a 1D CMUT array 2110, the acousticwindow 2120, the coupling medium 130, and the CMUT packaging substrate140. In this example, the 1D CMUT array 2110 includes multiple CMUTs 210a, 210 b, 210 c, . . . , 210 n, each of which may be a fully functionalCMUT transducer, as discussed above, such as with respect to FIG. 2. Theacoustic window 2120 varies along the elevation direction 2122 of the 1DCMUT array 2110, and all the CMUTs 210 may have the same acoustic windoweffect along a transducer array direction 2124 (or azimuth direction) asindicated in FIG. 21.

FIG. 22 shows the cross-section view of an alternative CMUT arrayapparatus 2200 (e.g., for a 1D, 1.5D. 1.75D or 2D array, etc.) includingan acoustic window 2220. The CMUT array apparatus 2200 includes a CMUTarray 2202, the acoustic window 2220, the coupling medium 130, a CMUTprotection layer 2204, and the CMUT packaging substrate 140. The CMUTarray 2202 includes multiple CMUTs (e.g. 2210 a, 2210 b, 2210 c, . . .). Each CMUT 2210 may include at least a common first electrode 2212 andan individual second electrode 2214 separated by a transducing space2216. For example, the common first electrode 2212 may be shared bymultiple CMUTs 2210 in the CMUT array 2202. Moreover, the commonelectrode 2212 may be connected to an electrical ground (GND) (not shownin FIG. 22). The common electrode 2212 may be designed to face thecoupling medium 130 and an individually addressed second electrode 2214of each CMUT 2210 may be shielded by the common electrode 2212 from thecoupling medium 130. On the packaging substrate 140, a pad 2218 may beprovided between the second electrode 2214 and the packaging substrate140 to connect to the second electrode 2214 of the CMUT 2210. As oneexample, one or more through-wafer interconnections may be fabricated toconnect the electrodes 2212, 2214 of the CMUTs 2210 from the top surfaceof the CMUT fabrication substrate 2218 to a bottom surface.

In some examples, the protection layer 2204 may be coated onto thesurface of the CMUTs 2210 to electrically isolate the CMUTs 2210 and thefirst electrode 2212 from the coupling medium 130. The protection layer2204 may be any suitable insulating or dielectric material, such aspoly(p-xylylene) (e.g., Parylene®), polyimide, oxide, nitride, RTV,urethane, polyurethane, non-conductive polymer, or other suitableplastic or rubber materials. The acoustic window 2220 may be any of theacoustic window configurations described herein.

FIG. 23 illustrates an example acoustic window 2320 having enhancedstructural performance according to some implementations. For example,in order to improve the mechanical properties of the acoustic window2320, one or more added structures may be introduced into the acousticwindow 2320 to provide greater structural rigidity and strength withminimum impact on the acoustic performance of the CMUT apparatus. In theillustrated example, the acoustic window 2320 includes a base portion2322 and one or more support structures 2324. The one or more supportstructures 2324 may be constructed from the same material as the baseportion 2322 or from a material that is different from the material ofthe base portion 2322. The example acoustic window 2320 illustrated inFIG. 23 includes at least two elongated parallel beams 2326. The baseportion 2322 may correspond to any of the examples of acoustic windowsdiscussed above. In the illustrated example, the base portion 2322includes an uneven surface, such as that described with respect to FIG.10, and the beams 2326 may conform to the shape of the base portion2322. Further, the beams 2326 may have a width w1, which may be uniform,or which may vary, and may have a height h, which may be uniform orwhich may vary.

In some examples, if a surface 2330 of the base portion 2322 having thebeams 2326 located thereon faces the coupling medium 130 (not shown inFIG. 23), the spaces surrounding the beams 2326 may or may not be filledwith a filling material. For example, in some cases, the coupling medium130 may fill the space surrounding the beams 2326, while in other cases,a separate filling material may fill the space. Alternatively, if thesurface 2330 and the beams 2326 of the acoustic window face outward fromthe transducer apparatus, then a suitable acoustic window material, asdiscussed above, may be selected to fill the space surrounding the beams2326, as discussed below with respect to FIG. 25.

FIG. 24 illustrates an example acoustic window 2420 having enhancedstructural performance according to some implementations. In thisexample, the support structure 2324 includes the one or more beams 2326discussed above, and one or more cross beams 2422. Further, as mentionedabove, when the surface 2330 (with the beams 2326, 2422) of the acousticwindow 2420 faces the coupling medium 130 (not shown in FIG. 24), thespace around the beams 2326, 2422 may or may not be filled with suitablefilling material. Alternatively, when the surface 2330 of the acousticwindow 2420 faces outward from the CMUT apparatus, then a suitablefiller material may be selected to fill the space around the beams 2326,2422.

FIG. 25 illustrates an example acoustic window 2520 having an enhancedstructural performance according to some implementations. In thisexample, the acoustic window 2520 may include a basic structure, asdiscussed above with respect to FIG. 24, including one or more beams2326 and one or more cross beams 2422. In this example, the spacesurrounding the beams 2326, 2422 is filled with a suitable fillingmaterial 2522. For example, the filling material 2522 may have anacoustic impedance that substantially matches the acoustic impedance ofthe material that the filling material 2522 contacts, i.e., either thecoupling material 130 or the target medium 828, as well as matching theacoustic impedance of the material of the base 2322.

FIG. 26 illustrates an example acoustic window 2620 having an enhancedstructural performance according to some implementations. The acousticwindow 2620 may include a portion of the structure described above withrespect to FIGS. 24 and 25; however, in this example, if the fillingmaterial 2522 is sufficiently strong mechanically, the base portion 2322may be eliminated from the acoustic window 2620, as shown in FIG. 26. Insuch a case, the acoustic window 2620 is made of the filling material2522 with beams 2326 and or 2422, or other support structures 2324embedded inside the filling material 2522. Typically, the supportstructures 2324 would be made of a material having a great mechanicalstrength or resistance to deformation than the filling material 2522,while the filling material 2522 may be selected based on acousticimpedance matching with mediums 130, 828 to be contacted. The supportstructures 2324 illustrated in FIGS. 23-26 are merely examples providedfor discussion purposes. Other suitable shapes of support structures2324 (i.e., hexagon or honeycomb, circles, etc.) may be used in otherimplementations, as will be apparent to those of skill in the art havingthe benefit of the disclosure herein.

The geometry of the support structure may be designed to maximize themechanical strength and reliability of the acoustic window, while havingminimum impact on the acoustic performance of the CMUT apparatus. Tominimize the acoustic impact of the enhanced structure, the width w1 ofthe beams 2326, 2422 may be less than 1.0 acoustic wavelength at thefrequency at which the CMUT operates. The height h of the beams 2326 maybe selected to provide a desired amount of mechanical strength for thewhole acoustic window. In some implementations, the materials of boththe enhanced structure 2324 and the filling material 2522 may beselected from the materials discussed above as being suitable for theacoustic window and the coupling medium in some implementations. Theacoustic windows with enhanced structures 2324 may be used for acousticwindows having any of the shapes or thickness profiles disclosed herein.

Example Process

FIG. 27 is a flow diagram illustrating an example process for a CMUTwith an acoustic window according to some implementations.

At 2702, one or more CMUTs are positioned to have an operationaldirection facing an acoustic window. For example, the acoustic windowmay be constructed of a material suitable to contact a target medium.Thus, the acoustic window may have mechanical properties sufficient towithstand contacting the target medium and may have acoustictransmission properties similar to those of the target medium. Further,in some cases, the acoustic window may include a focusing capability,such as an acoustic lens for focusing acoustic energy on a focallocation in the target medium. Alternatively, or additionally, the CMUTmay include a focusing capability. Additionally, the acoustic window mayinclude one or more features to reduce or minimize acoustic reflection,such as one or more patterns, coatings, structured layers, trenches, orthe like, as described above. The acoustic window may further includeone or more structural enhancements to improve the mechanical propertiesof the acoustic window.

At 2704, a coupling medium is provided between the one or more CMUTs andthe acoustic window. For example, the coupling medium may be of amaterial having acoustic properties similar to those of the acousticwindow and/or the target medium. The coupling medium may be enclosed orretained by a housing or the like. In some cases, the coupling mediummay provide a focusing capability based, at least in part, on aconfiguration of the acoustic window.

At 2706, the one or more CMUTs are operated in at least one of: areception mode to receive acoustic energy through the acoustic windowand the coupling medium; or a transmission mode for transmittingacoustic energy from the one or more CMUTs through the coupling mediumand the acoustic window. For example, the one or more CMUTs may beoperated to transmit acoustic energy as an acoustic beam through theacoustic window when the acoustic window is placed into contact with atarget medium. The acoustic window may include one or more features tominimize acoustic reflection of the acoustic beam. Further, in someexamples, the acoustic window may be designed to focus the acousticenergy toward a focus location.

Additionally, the example process described herein is only one exampleof a process provided for discussion purposes. Numerous other variationswill be apparent to those of skill in the art in light of the disclosureherein. Further, while the disclosure herein sets forth several examplesof suitable apparatuses and environments for executing the process,implementations herein are not limited to the particular examples shownand discussed.

Furthermore, this disclosure provides various example implementations,as described and as illustrated in the drawings. However, thisdisclosure is not limited to the implementations described andillustrated herein, but may extend to other implementations, as would beknown or as would become known to those skilled in the art. Reference inthe specification to “one implementation,” “this implementation,” “theseimplementations” or “some implementations” means that a particularfeature, structure, or characteristic described is included in at leastone implementation, and the appearances of these phrases in variousplaces in the specification are not necessarily all referring to thesame implementation.

CONCLUSION

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described. Rather,the specific features and acts are disclosed as example forms ofimplementing the claims.

1. A method comprising: forming one or more capacitive micromachinedultrasonic transducers (CMUTs) on a CMUT substrate, the one or moreCMUTS formed to have an operational direction facing away from the CMUTsubstrate; disposing a coupling medium over the one or more CMUTs andthe CMUT substrate; and disposing an acoustic window over the couplingmedium, the one or more CMUTs, and the CMUT substrate, wherein thecoupling medium couples acoustic energy between the one or more CMUTsand the acoustic window.
 2. The method as recited in claim 1, wherein:the forming the one or more CMUTs on the CMUT substrate comprisesforming the one or more CMUTs to include a flexible membrane as an outersurface of the one or more CMUTs; and the flexible membrane is adjacentto the coupling medium following disposing of the coupling medium overthe one or more CMUTs and the CMUT substrate.
 3. The method as recitedin claim 2, further comprising forming a protection layer over the oneor more CMUTs, wherein the protection layer is disposed between theflexible membrane and the coupling medium to electrically insulate, atleast in part, the one or more CMUTs from the coupling medium.
 4. Themethod as recited in claim 2, further comprising: forming an electrodeon the flexible membrane; and forming a protection layer over the one ormore CMUTs, wherein the protection layer is disposed between theelectrode and the coupling medium to electrically insulate, at least inpart, the one or more CMUTs from the coupling medium.
 5. The method asrecited in claim 1, further comprising disposing the acoustic windowover the coupling medium after disposing the coupling medium over theone or more CMUTs and the CMUT substrate.
 6. The method as recited inclaim 1, further comprising disposing the acoustic window over the oneor more CMUTs and the CMUT substrate before disposing the couplingmedium over the one or more CMUTs and the CMUT substrate.
 7. The methodas recited in claim 1, wherein the forming the CMUTs on the CMUTsubstrate comprises forming a plurality of CMUT elements to form anarray of CMUT elements on the CMUT substrate.
 8. The method as recitedin claim 7, further comprising forming the array with a curve to focusemitted acoustic energy on a focal location and/or to achieve a desiredacoustic beam profile.
 9. The method as recited in claim 1, furthercomprising: disposing the CMUT substrate on a packaging substrate; andforming a seal between a housing and the packaging substrate to containthe coupling medium.
 10. The method as recited in claim 1, furthercomprising: constructing the acoustic window of a first material; andconstructing the coupling medium of a second material having an acousticproperty different from the first material.
 11. A capacitivemicromachined ultrasonic transducer (CMUT) apparatus comprising: a CMUTsubstrate; one or more CMUTs formed on the CMUT substrate to have anoperational direction facing away from the CMUT substrate; an acousticwindow, the acoustic window disposed over the one or more CMUTs and theCMUT substrate to pass acoustic energy to or from the one or more CMUTsin the operational direction; and a coupling medium disposed between theone or more CMUTs and the acoustic window to couple the acoustic energybetween the one or more CMUTs and the acoustic window.
 12. The CMUTapparatus as recited in claim 11, wherein: the one or more CMUTs includea flexible membrane as an outer surface of the one or more CMUTs; andthe flexible membrane is adjacent to the coupling medium.
 13. The CMUTapparatus as recited in claim 12, further comprising a protection layerdisposed over the one or more CMUTs, wherein the protection layer isdisposed between the flexible membrane and the coupling medium toelectrically insulate, at least in part, the one or more CMUTs from thecoupling medium.
 14. The CMUT apparatus as recited in claim 12, furthercomprising: an electrode on the flexible membrane; and a protectionlayer disposed over the one or more CMUTs, wherein the protection layeris disposed between the electrode and the coupling medium toelectrically insulate, at least in part, the one or more CMUTs from thecoupling medium.
 15. The CMUT apparatus as recited in claim 11, wherein:the acoustic window is constructed of a first material; and the couplingmedium is constructed of a second material having an acoustic propertydifferent from the first material.
 16. A capacitive micromachinedultrasonic transducer (CMUT) apparatus comprising: a CMUT substrate; aplurality of CMUT cells formed on the CMUT substrate to have anoperational direction facing away from the CMUT substrate, wherein theplurality of CMUT cells are configured as an array; an acoustic window,the acoustic window disposed over the one or more CMUTs and the CMUTsubstrate to pass acoustic energy to or from the one or more CMUTs inthe operational direction; and a coupling medium disposed between theone or more CMUTs and the acoustic window to couple the acoustic energybetween the one or more CMUTs and the acoustic window.
 17. The CMUTapparatus as recited in claim 16, wherein: the CMUT cells includerespective flexible membranes as an outer surface of the CMUT cells; andthe flexible membrane is adjacent to the coupling medium.
 18. The CMUTapparatus as recited in claim 16, wherein the array includes a pluralityof CMUT elements, each element including a group of the CMUT cells. 19.The CMUT apparatus as recited in claim 18, wherein at least some of theCMUT elements share a common first electrode and have independentlyaddressable electrodes as respective second electrodes.
 20. The CMUTapparatus as recited in claim 16, wherein: the acoustic window isconstructed of a first material; and the coupling medium is constructedof a second material having an acoustic property different from thefirst material.